The present invention relates to a method for guiding a boom as set forth in the preamble of the appended claim 1, as well as a system for guiding a boom as set forth in the preamble of the appended claim 8.
Lifting devices are used, for example, for transferring loads in various loading situations. Such lifting devices are often provided with a boom, by which the extension of the lifting device can be improved, compared with fixed lifting devices. Furthermore, such a boom is normally mounted on its support frame in such a way that the boom has at least two degrees of freedom. For example, the boom is arranged to be rotatable in relation to its support frame, and in addition to this, the angle of inclination of the boom can be adjusted in the vertical direction. Furthermore, the boom is often formed of two or more boom parts arranged to be movable in relation to each other, which improves the use of the boom for the transfer of loads. The boom parts may be formed in such a way that, if necessary, they move within each other, i.e. they form a so-called telescopic structure, or in such a way that the boom parts are connected to each other in an articulated manner, wherein the angle between the boom parts can be adjusted in at least one direction. Various combinations of the above-mentioned structures are also known.
Furthermore, the boom is typically provided with a lifting means, by which the load to be handled can be gripped. This lifting means can be, for example, a hook, a lifting fork, or the like. This lifting means is preferably either fixed to the boom in an articulated manner, or the lifting means is fixed to a lifting gear by means of a wire rope or the like.
At the stage when the required transfer work is to be carried out, the boom is guided so that the load to be transferred can be fixed to the lifting means. After this, by controlling the movements of the boom, the actual hoisting and transfer work is carried out to transfer the load to a desired location. Due to certain physical factors, such as mass inertia, this hoisting and transfer stage has various undesired force effects on the boom, such as jerks and oscillation. These force effects may cause a jerking motion in the boom, bouncing of the goods to be transferred, etc. These undesired force effects are directed, for example, to the boom control means, such as control cylinders, and may cause early defects in these control means. Furthermore, the fastenings of the boom may be impaired and cause danger situations. Thus, when transferring the load, these jerking and bouncing movements should be avoided, wherein when moving the load, the person operating the lifting device must see that too quick hoisting and transfer movements are not performed. Thus, it is not possible to use the maximum hoisting and transfer speed when moving loads.
Some solutions have been developed to reduce undesired force effects on the boom. For example, U.S. Pat. No. 4,815,614 presents a boom control system, in which the pressure of the boom lifting cylinder is monitored and when the pressure exceeds a certain limit, the maximum opening of the control valve of the lifting cylinder, i.e. the maximum speed of the cylinder, is limited linearly in relation to the increase in the pressure. This system is intended to limit the maximum speed of the boom in the range critical in view of the loads, and it is primarily intended to protect the boom.
Japanese patent publication JP 5-321297 presents an active dynamic power feedback for controlling the cylinder of an excavator boom. This is implemented by means of a separate electrical hydraulic system. In this system, the dynamic force and position have a 180xc2x0 phase difference from each other. Thus, the power feedback attenuates the oscillation of the boom. The dynamic force is calculated by high-pass filtering the feedback signal formed from the pressures of the lifting cylinder. The gain of the feedback is a function formed on the basis of the static force of the cylinder. The static force is obtained by low-pass filtering the power feedback. Thus, this is an active method for attenuating the oscillation.
U.S. Pat. No. 5,638,267 discloses a method and a device for avoiding undesired dynamic phenomena in a closed control circuit. In the method presented in this publication, a series of impulses is formed, which is advantageous in view of avoiding oscillation and which is used to adjust the control by the control device. This series of impulses can be generated by dividing the original control into smaller units with a given amplitude and time difference. These amplitudes and time differences are obtained by minimizing a specific oscillation equation. This method, however, for calculating the control, uses precalculated conditions and formulas in such a way that this method is not applicable for the handling of variable loads in real time.
It is an aim of the present invention to provide a method for controlling a boom and a system for controlling a boom, in which the aim is to make the load transfer rate as high as possible, but without causing oscillation, jerks or other undesired force effects on the boom to a significant degree. The invention is based on the idea of forming at least second degree filtering functions, whose properties, such as the frequency band, are dynamically adjusted on the basis of control variables obtained from sensors. The method according to the present invention is primarily characterized in that the filtering applies a filtering function of the second or a higher order, comprising at least one parameter that can be set, and that the method further comprises at least the following steps: a determining step to determine at least one specific oscillation frequency of the boom on the basis of said measurement signals; a setting step, in which at least one specific oscillation frequency determined in the determining step is used to set at least one parameter for a filtering function; a filtering step, in which said control signal is filtered with said filtering function to form a control signal; and a control step, in which said control signal is used to guide the movement of the boom. The boom control system according to the present invention is primarily characterized in that the filter is a filter of the second or a higher order, whose at least one parameter is arranged to be set, and that the control system further comprises at least determining means for determining at least one specific oscillation frequency of the boom on the basis of said measurement signals; setting means for setting at least one parameter for a filtering function on the basis of at least one specific oscillation frequency determined at the determining step; and control means for controlling the movements of the boom with said control signal.
By means of the present invention, considerable advantages are achieved when compared to methods and boom control systems of prior art. By the method of the invention, the acceleration and deceleration of the boom can be increased more than is possible in prior art systems, but without causing harmful force effects on the boom. Thus, the lifting and transfer work can be speeded up, reducing the time required for the lifting and transfer work. As the harmful force effects on the boom are significantly reduced, also the need for maintaining and repairing the boom will reduce and its service life will be prolonged. Moreover, danger situations cannot occur as easily as in prior art solutions.